High power laser diodes may be used for, e.g., optical communications, printing, generation of light by second harmonic generation, and medical applications. They are frequently used in conjunction with optical fibers. For many applications, the laser is required to operate in a single transverse optical mode. A single transverse-mode laser diode may have, for example, a 1 μm×5 μm wide output aperture and typically may provide output power up to about 0.4 W. A narrow aperture is needed to produce a single transverse optical mode. However, a very high power density may exist at the aperture, which is a limiting factor for single-transverse mode laser diodes. Alternatively, a multi-mode laser diode may have, for example, a 1 μm×100 μm wide output aperture and operate at output powers up to about 4 W. However, a multi-mode laser does not produce a single “spot” of light, but instead produces a series of spots, which is difficult to focus into a small area such as the end of an optical fiber. See generally Heterostructure Lasers, Part B: Materials and Operating Characteristics, H. C. Casey, Jr., and M. B. Panish, Academic Press, 1978, pp. 252-254.
The maximum output power that a laser diode may reliably provide is generally limited by damage to the semiconductor material caused by the high optical power density in the optical cavity. Often this damage first occurs at a facet mirror, and is referred to as Catastrophic Optical Damage (COD). It is well known that COD may limit power density at laser diode facets. See generally Reliability and Degradation of Semiconductor Lasers and LEDS, Mitsuo Fukuda, Artech House, 1991, pp. 128 -136; COD occurs because crystal defects at the facet absorb light during laser operation and thereby heat the facet. When the semiconductor comprising the laser diode is heated, the band gap energy is reduced, making the absorption even stronger, which in turn heats the facet even more and a runaway effect takes place thereby leading to COD. This typically causes the material near the facet to melts and/or ablate. Known ways to minimize this runaway effect include sophisticated techniques for coating the facets (see U.S. Pat. No. 5,144,634) or using a semiconductor other than aluminum containing materials (such as AlGaAs), e.g., GaInP (see U.S. Pat. No. 5,389,396), which is aluminum-free and oxidizes less, causing fewer defects at the facet. But at high output power, reliability is still compromised, limiting output power to about 4 W for a 100 μm wide multi-mode laser, or 400 mW for a single transverse mode laser diode.
Thermal effects may also limit the output power of a single-transverse-mode laser diode. At high input current levels, heating of the laser's active stripe region increases the index of refraction relative to the adjacent regions. This typically causes the laser waveguide to operate higher-order optical modes, which is unacceptable for many applications.